Seismic site effects for the Sacramento-San Joaquin Delta

The Sacramento-San Joaquin Delta is a critical link in California's freshwater delivery system, home to a unique ecosystem, and an area experiencing increasing urbanization pressures. The Delta consists of a complicated network of channels and reclaimed islands that collects water from about 40% of California's surface area and directs it to San Francisco Bay. There are about 1700 km of levees and over 60 islands with ground surface levels up to 8 m (25 feet) below adjacent water ways (California 1992). Levee failures during an earthquake are a major concern because rapid inundation of the inner islands has the potential to significantly reduce the freshwater supply for California, in addition to damaging the natural habitat, crops and civil infrastructure. The California Department of Water Resources performed preliminary seismic stability evaluations of the levee system and concluded that the seismic wave amplification at sites underlain by highly organic soils was a major source of uncertainty in seismic hazard evaluations for the delta (California 1992). A series of laboratory tests for peat and highly organic soils are performed and regression models and correlations between residuals are presented. The regression models of dynamic properties for inorganic soils and those correlations between residuals are also presented. The source of uncertainties from input motions, soil profiles, and estimation of dynamic properties are propagated through Monte Carlo simulations for one dimensional site response analysis. One dimensional and two dimensional site response analyses are performed for three profiles in the Sacramento-San Joaquin Delta with more than two hundred input ground motions, and then the 2D/1D correction factors of spectral acceleration at levee crests are obtained. The regression models of stress reduction factors for 1D analyses and seismic coefficient reduction factors for 2D analyses are obtained with the characteristics of the spectral accelerations at levee crests. The regression models of Newmark sliding block displacements are presented and displacement seismic hazard curves at the Sacramento-San Joaquin Delta are presented.